This invention relates generally to polyester containers and more particularly to a polyester container formed of polyethelene terephthalate (PET) and provided with an improved base configuration enabling the container to be filled with hot liquid without deformation.
Typical PET containers are formed by a process in which an elongated tubular preform, made by injection molding or other process, is heated and expanded into conformity with the inside surface of a mold cavity, thus producing a semi-rigid thin-walled container. Since the container is exposed to various pressures and forces during production and use, as will better be explained below, it must be designed to respond to such physical influences while maintaining the predetermined and desired configuration. Random or asymmetrical buckling and deformation of the container produce an aesthetically and commercially unacceptable product and therefore, must be avoided.
During the process of making a blow molded PET container, the preform is typically stretched and inflated so as to impart both axial and radial elongation in the material. In the art, such forming is known as biaxial elongation. The biaxial elongation imparts retractive stresses within the material which, if not relaxed or physically restrained, tend to cause the article to shrink and deform in the directions of the previous elongation.
The influence of such unrelaxed retractive stresses is particularly significant during certain stages of the production process. The first occurs immediately after demolding the container. At this stage, the elevated temperature of the container material results in an article which is significantly less rigid than the final product. Predictably, the retractive stresses tend to have a greater influence during this phase of the production process where the "memory" of the PET tends to cause the container to attempt a return to its original preform shape.
Another phase of the production process where retractive stresses are significant occurs in the case of a so-called "hot-fill" container, where a beverage or product at an elevated temperature is deposited in the container. The elevated temperature of the beverage imposes additional mechanical stresses on the container structure. Immediately after a hot liquid is dispensed into the container, the liquid's temperature decreases the rigidity of the PET material and again, the container is susceptible to the effects of the unrelaxed retractive stresses.
Beyond the production process, the container must also be structured so that it will sustain internal pressure changes while maintaining its desired configuration. For example, as a hot-filled liquid cools, it shrinks in volume resulting a negative pressure being produced within the container. During use, the container must be resistant to deformation caused by a sudden increase in internal pressure as can occur when the container is handled or dropped.
It is known within the industry that the PET in the bottom wall of the container will not be molecularly consistent, with regard to molecular orientation, throughout a cross-section of the base. Rather, the bottom wall will consist of an amorphous region in the center of the base, where the PET is significantly thick and not stretched or biaxially oriented by the blowing process, and a uniformly oriented region, where the PET is stretched and biaxially oriented, adjacent to the peripheral edge connecting the base to the side wall of the container. Both of the above mentioned regions are resistant to the retractive stresses; the center amorphous region due to its increased thickness and the uniformly oriented region because of its biaxial orientation. However, between the amorphous and uniform regions there exists a transition or neck region that is not significantly thick or uniformly oriented, and therefore, not resistive to the retractive stresses.
U.S. Pat. No. 4,598,831, issued to Nakamura, discloses a method of reshaping and orienting a portion of the transition region in the base to form a radial array of triangular pyramid sections, the bottom surface of which is sufficiently stretched and oriented. However, in the above mentioned patent a significant amount of the transition region remains transitional, and therefore unoriented and non-resistive to the retractive stresses.
In accordance with the present invention, a PET container is provided having a base structure of an improved configuration which maintains structural rigidity and resistance against random deformation and shrinkage in response to the previously described mechanical and thermal stresses. This is accomplished by providing spiral array of reinforcing embossments, ribs or other structural interruptions in the region spanning the transitional area of the base between the amorphous PET and the uniformly oriented portions of the base.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.